Fig 1.
Each animal was scanned twice, separately in the presence of either bacon- or chow-paired cues, but without either food stimulus. Animals were conscious during the uptake phase, and immediately prior to and during acquisition (20 min scan), animals were anesthetized with continuous isoflurane. During the 2-day break, CPP was maintained with one conditioning session per day (not shown).
Fig 2.
Gastric bypass effectively reduced body weight following diet-induced obesity.
Pre-operative body weights during diet-induced obesity (establishment phase) are shown in the shaded region. Weight loss following surgery, specific to the RYGB animals, is shown in the non-shaded region, with a more pronounced difference in the Q2/3/4 sub-group. The inset shows the differences in body weights, following group-assignment, the day before experimental or sham surgeries (*p< 0.05 compared to Sham-ND; no difference between Q1 and Q2/3/4 before surgery, not shown).
Fig 3.
Objective health-screen and bypass outcome.
As gross health and weight were highly variable in the experimental group, compared to the sham-operated controls, relative weight-loss after gastric bypass was calculated and used to objectively determine these animals’ health. (A) The 7 RYGB animals with the lowest and most stable weight-losses (Q1) were compared to the remaining 75% (**p< 0.01 compared to each other; Sham-AL shown as reference). (B) This objective separation was valid, as differences in mortality were explained by the screen.
Fig 4.
Weekly food and water intake during bacon- and chow-conditioning.
Weekly food (A) and water (B) intakes were calculated relative to CPP Weeks 1 and 2, and these values, expressed in calories and volume consumed, respectively, were adjusted based on body-weight (*unadjusted p < 0.001 versus Sham-AL,-PF and-ND; ~unadjusted p < 0.001 versus Sham-PF and-AL; #unadjusted p < 0.007 versus all groups).
Fig 5.
Healthy RYGB rats (Q1) showed significant bacon CPP.
To quantify preference for the bacon-paired chamber, the amount of time each animal spent in the bacon-paired chamber was standardized to total time in both chambers. There was a bypass-specific increase in bacon-chamber preference after conditioning (Test Days 1 and 2) relative to Habituation Day, yet importantly, this behavioral response was strictly observed in the RYGB animals that responded well to the surgery (*unadjusted p < 0.05 compared to Habituation Day).
Fig 6.
Food intake throughout the conditioning sessions.
Animals were given 10-minutes to consume 5-grams of either bacon or chow during the final 10 min of each conditioning session. Irrespective of group, animals consumed more bacon than chow throughout the conditioning-component, suggesting that relative to chow, bacon was highly palatable. Cumulative intake is the sum grams consumed between CPP days 1 and 12 for each food-stimulus (arrows indicate Test Days 1 and 2; thick line: p < 0.05 for bacon versus chow on the given days; *p < 0.05 for bacon versus chow in a specific group). The only significant differences observed were found between bacon and chow (Sham-ND and Sham-PF).
Fig 7.
Statistical Parametric Mapping (SPM) showing regional brain activation to bacon-paired cues in rats that underwent gastric bypass surgery (RYGB; blue) or sham-operated controls fed a HF diet (green).
SPM clusters were overlaid onto a magnetic resonance imaging atlas of the rat brain set to stereotaxic coordinates and significant clusters were outlined in the Paxinos rat brain atlas. (RS-retrosplenial cortex; V1M-primary visual cortex; IC-inferior colliculus; MPB-medial parabrachial nucleus; DMTg-dorsomedial tegmental area; Sim/SimB- simple lobule; CB(8)-cerebellum lobule 8).
Fig 8.
Correlation plots between CPP preference for Chow or Bacon and BGluM during Habituation (Hab), Test Day 1 (TD1) or TD2.
Table 1.
SPM Results.